Optical fibers have been adapted to send information or energy along circuitous paths and over long distances. This is possible because the optical fibers are provided with a cladding which greatly reduces attenuation of the optical signal by internally reflecting incident light. Attenuation has, however, been employed to adapt optical fibers as sensors. To this end, the cladding is removed from a length of the optical fiber which is bent one or more times in the unclad length. A bend results in an increased angle of incidence and this together with the absence of cladding causes a portion of the incident light to be transmitted into the surrounding medium rather than being essentially totally reflected. The amount of light transmitted into the surrounding medium is dependent upon the physical characteristics of the surrounding medium and this phenomenon has been employed to determine a characteristic of the surrounding medium such as by determining concentration or detecting the presence of a particular substance in the surrounding media. This type of testing requires that the stripped section of optical fiber physically contact the medium to produce the required attenuation indicative of the presence of the substance. Unfortunately, substances such as oil tend to coat the stripped surface after initial contact and it is this interface which determines the degree of attenuation. As a result these devices are unsuitable for long term monitoring unless the probe is frequently removed for cleaning of contaminants and the accuracy of the device can be compromised by contaminants during the periods between cleanings.